This project focuses on how antigens are processed in the intestine of mice. While it is clear that the outcome of oral antigen exposure can be either positive, i.e., the development of mucosal IgA responses, and in some cases the induction of systemic immunity as well, or negative, i.e., the induction of oral tolerance, the details of why one or the other outcome occurs is complex and poorly understood. While it is known that the antigen formulation, the presence of adjuvants, and the antigen dose, as well as genetic factors, can affect mucosal immune responses, how these act to influence immunity has never been established. In prior studies we have established the presence of different antigen-presenting cell populations in the Peyer's patch (PP) and lamina propria and have detailed the surface phenotype, function, and migration of DCs in the PP using in situ immunofluorecense microscopy and in situ hybridization, flow cytometry of purified cells, and in vitro assays of cytokine production (ELISA and quantitative RT-PCR) and T cell differentiation. We determined that there are 3 separate subpopulations of immature DCs in the PP, lymphoid (CD8+), myeloid (CD11b+), and double negative (DN) Dcs that express neither CD8 or CD11b. These separated DC subpopulations are located in distinct sites in the PP, and are capable of inducing the differentiaion of T cells into cells that produce unique cytokine profiles. Most importantly, we demonstrated that PP DCs have the unique capacity to induce the differntiation of T cells that produce high levels of IL-10, a cytokine important for the IgA B cell differentiation. These studies thus are some of the first to directly demonstrate that DCs from different tissues may be unique in their ability to induce tissue specific immunity. We have also recently demonstrated that DN DCs in the subepithelial dome region of the PP process viral antigen from virally infected apoptotic epithelial cells. During the past year we have made several advances, in collaboration with other NIH and outside investigators. 1) We identified a genetic major locus for the susceptibility of colitis in the Gi2a-/- mouse model of intestinal inflammation. This happens to be the same locus identified by others in the IL-10-/- model providing strong evidence that this locus is important. 2) We demonstrated that dendritic cells from germ-free mice are phenotypically and functionally normal. which demonstrates that commensal bacteria are not required for the generation of functionally normal dendritic cells. 3) Demondtrated the lack of a role for TL antigen in the activation of intraepithelial lymphocytes.